1. Field of the Invention
The present invention pertains to fuel cells, and, more particularly, to a cell maintenance device for fuel cell stacks.
2. Description of the Related Art
Fuel cell technology is an alternative energy source for more conventional energy sources employing the combustion of fossil fuels. A fuel cell typically produces electricity, water, and heat from a fuel and oxygen. More particularly, fuel cells provide electricity from chemical oxidation-reduction reactions and possess significant advantages over other forms of power generation in terms of cleanliness and efficiency. Typically, fuel cells employ hydrogen as the fuel and oxygen as the oxidizing agent. The power generation is proportional to the consumption rate of the reactants.
A significant disadvantage which inhibits the wider use of fuel cells is the lack of a widespread hydrogen infrastructure. Hydrogen has a relatively low volumetric energy density and is more difficult to store and transport than the hydrocarbon fuels currently used in most power generation systems. One way to overcome this difficulty is the use of “fuel processors” or “reformers” to convert the hydrocarbons to a hydrogen rich gas stream, which can be used as a feed for fuel cells. Hydrocarbon-based fuels, such as natural gas, LPG, gasoline, and diesel, require conversion for use as fuel for most fuel cells. Current art uses multi-step processes combining an initial conversion process with several clean-up processes. The initial process is most often steam reforming (“SR”), autothermal reforming (“ATR”), catalytic partial oxidation (“CPOX”), or non-catalytic partial oxidation (“POX”). The clean-up processes are usually comprised of a combination of desulphurization, high temperature water-gas shift, low temperature water-gas shift, selective CO oxidation, or selective CO methanation. Alternative processes include hydrogen selective membrane reactors and filters.
Thus, many types of fuels can be used; some of them hybrids with fossil fuels, but the ideal fuel is hydrogen. If the fuel is, for instance, hydrogen, then the combustion is very clean and, as a practical matter, only the water is left after the dissipation and/or consumption of the heat and the consumption of the electricity. Most readily available fuels (e.g., natural gas, propane and gasoline) and even the less common ones (e.g., methanol and ethanol) include hydrogen in their molecular structure. Some fuel cell implementations therefore employ a “fuel processor” that processes a particular fuel to produce a relatively pure hydrogen stream used to fuel the fuel cell.
One problem arising in proton exchange membrane (“PEM”) fuel cells used with fuel processors is the formation of hydrogen peroxide on the platinum catalyst of precious metals catalyzed reactors. One mode of decay in PEM fuel cells is due to the formation of hydrogen peroxide at the anode of the fuel cell. The hydrogen peroxide currently limits PEM fuel cell life. This mechanism was first elucidated by A. B. LaConti in the 1960's. See “Mechanisms of membrane degradation (polymer electrolyte membrane fuel cells and systems, PEMFC)”, A. B. LaConti, M. Hamdan, and R. C. McDonald, Handbook of Fuel Cells; Vol. 3, Ch 49, pp 647-662, Edited by W. Vielstich, A. Lamm, and H. Gasteiger, Wiley, Chichester UK, 2003. Hydrogen peroxide is a strong oxidant that attacks the fuel cell membrane. It is generally formed by oxygen diffusing from the fuel cell cathode to the anode. The graph in FIG. 1 shows that even with very low oxygen partial pressure, the peroxide partial pressure can be quite high at anode potentials (˜0.0 volts). It is apparent from FIG. 1 that, if the anode potential is raised 200 millivolts, the peroxide concentration drops by three orders of magnitude.
More particularly, platinum oxide is a relatively inactive catalyst for oxygen reduction. Platinum forms a hydrated oxide Pt(OH)2 according to the equilibrium relation:
The equilibrium potential for forming the hydrated oxide is given by:Eo=0.98−0.0591 pHAtlas of Electrochemical Equilibria in Aqueous Solutions (2nd ed), M. J. N. Pourbaix, NACE, Houston, Tex. 1974, page 379 (“Pourbaix”). Pourbaix also shows how higher oxides may form. In addition, Pourbaix shows a complex relationship between Pt, PtO, PtO2 and Pt2+ ion.
The result of these reactions and relations are shown in FIG. 2, which is modified from Pourbaix. FIG. 2 shows the domains of immunity where Pt does not corrode and the domain of passivation where Pt corrodes to a stable hydrated oxide. The oxides are not especially catalytic for oxygen reduction. An inspection of FIG. 2 shows that dropping the cathode potential below the solid line 200, will make the platinum oxide unstable, and make the metal stable. If the cathode potential is allowed to move above the line 200, the oxide will be stable and the metal will be unstable. The process is also complex because the local pH has an effect on the process as well. Increasing the pH, as might occur under high current conditions (e.g., from cathodic proton consumption), will favor the formation of the oxide at lower cathode potentials.
These and other, similar problems have been known for more than 40 years. During this time, the art has sought to find a technically feasible, economically viable solution to these challenges. The problem is exacerbated by competition from alternative technologies, which are already capable generating and providing power at extremely low costs, due in part to an already installed infrastructure. Several approaches have been proposed, but none have found commercial acceptance.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.